1. Field of the Invention
The present invention relates to a tire having minimized rolling resistance and to a mould for producing said type.
2. Description of the Related Art
Generally, a tire for a motor vehicle is made from a predetermined viscoelastic material and comprises at least one casing ply, right- and left-hand beads, a tread strip placed on the crown of said casing ply, and at least one belt strip interposed coaxially between said casing ply and said tread strip. The casing ply has a profile (plyline), in the plane of a meridian section, which has a central crown (or under-belt) portion and two sidewall portions, one on the right and one on the left.
The major source of energy dissipated by a tire when it rolls on a road surface consists of hysteresis losses due to the viscoelastic materials from which it is made. In particular, the energy dissipated by the various parts of a tire depends on the cyclical stresses and deformations to which it is subjected by the continual alternation of the inflated configuration (distant from the area of contact with the road or footprint) and of the flattened configuration (centre of the footprint area).
It is estimated that the hysteresis losses account for 90-95% of the total energy dispersed by a tire and that the remaining 5-10% is attributable to other dissipation mechanisms, such as the slip between the tire and the road, the aerodynamic losses due to the friction of the air, and the internal friction between the air and the tire.
Most of the energy is dissipated by the tread strip of the tire (xe2x89xa750%). This energy dissipation is essentially due to the fact that the assembly consisting of the tread strip and the belt package (belt strips) of the tire undergoes a change of curvature in both the longitudinal direction (inflection due to the passage through the footprint) and the meridian direction (flattening of the tread strip).
This energy dissipation produces the rolling resistance of the tire, and therefore the term xe2x80x9crolling resistancexe2x80x9d (R.R.) will be used in the course of the present description and in the claims to denote the power dissipated in one cycle as a result of the cyclical deformations of the viscoelastic materials of the tire in neutral, in other words when it is not subject to a torque.
The object of the present invention is to reduce the power dissipation of a tire when it rolls on a road surface, and consequently to reduce its rolling resistance.
The inventors have found that the power dissipated by a tire can be reduced by minimizing the deformations undergone by the assembly consisting of the tread strip and belt package of the tire, in such a way as to contain the quantity of power dissipated in these. They have also found that a reduction in dissipation can be obtained by shifting the power dissipation of the tread strip to the area of the sidewalls, in such a way as to reduce the ratio between the quantity of power dissipated in the tread and the quantity of power dissipated in the sidewall.
The inventors have also identified a mould which enables a tire to be produced with the desired characteristics.
A first aspect of the invention is a tire for a motor vehicle, made from a predetermined viscoelastic material, and comprising
a) at least one casing ply,
b) a tread strip placed on the crown of said casing ply,
c) at least one belt strip interposed coaxially between said casing ply and said tread strip,
d) right-hand and left-hand sidewalls, and
e) right-hand and left-hand beads,
f) said casing ply having a profile (plyline), in the plane of a meridian section, which has a central crown (under-belt) portion and two sidewall portions, one on the right and one on the left,
g) each of said sidewall profile portions being delimited by two points S and K, where the point S is located substantially at one edge of said at least one belt strip and the point K separates a sidewall from a bead,
h) said tire assuming, at the operating pressure and in the absence of a load, an inflated configuration having a predetermined outer profile which matches a predetermined enclosing rectangle,
i) said outer profile having a predetermined maximum chord {overscore (C)} and a predetermined maximum height {overscore (H)}, the ratio {overscore (H)}/{overscore (C)} lying in a range from 0.6 to 0.8, characterized in that,
j) in said inflated configuration, said crown profile portion has a radius of meridian curvature xcfx81c lying in a range from 406 mm to 690 mm and each of said sidewall profile portions forms an angle xcex1s with the axis of rotation of the tire, at said point S, which lies in a range from 25xc2x0 to 30xc2x0.
Preferably, each of said sidewall profile portions forms an angle xcex1k with the axis of rotation of the tire, at said point K, which is xe2x89xa745xc2x0.
Advantageously, said crown profile portion is substantially flat.
In the inflated configuration, the tire according to the invention has, for given overall dimensions, extremely flat belts in the meridian plane, so that in the flattened state, when the belts pass through the footprint, their deformation in this plane is virtually zero.
Moreover, the median casing profile has a more erect (vertical) inflated configuration in the lower area of the sidewall than a conventional tire, and consequently the deformations in the upper parts of the sidewalls are greater than those in a conventional tire.
In the tire according to the invention, therefore, the power dissipation has an optimized distribution among the various parts because it is more balanced than that of a conventional tire. This makes it possible to reduce the total power dissipation and minimize the rolling resistance of the tire.
The shape of the tire in the inflated state having the structural characteristics indicated above is obtained by means of a special geometry of a mould used for forming it.
A second aspect of the invention is a mould for producing a tire made from a predetermined viscoelastic material, said mould having
A) a crown formed by radially movable sectors for moulding a tread strip and shoulders of said tire, and
B) cheeks for moulding sidewalls and beads of said tire,
C) said mould having a profile, in the plane of the meridian section, which has a predetermined maximum chord C, a predetermined maximum height H and a fitting line I, and is formed centrally by a base profile of one sector and laterally by profiles of said cheeks,
D) said base profile of the sector having a central portion flanked by two connecting portions, which in turn are flanked by two lateral portions,
E) each cheek profile having a total height Hg and having a first portion with a first radius of curvature Rfs and a second portion with a second radius of curvature Rfi, characterized in that
F) the ratio between said first and second radius of curvature Rfs/Rfi ranges from 0.45 to 0.56.
Preferably, the ratio between said first and second radius of curvature Rfs/Rfi is approximately 0.5.
Advantageously, the centres of said first and second radius of curvature Rfs and Rfi lie on said maximum chord C and said maximum chord C is located at a distance Hlc from said fitting line I equal to approximately ⅔ of said height of the cheek profile Hg.
In one embodiment, said central portion of said base profile of the sector is substantially flat and has a radius of meridian curvature Rxe2x89xa7500 mm, and each of said connecting portions has, at the point of junction with one of said lateral portions, an angle xcex10 with respect to the longitudinal axis of the mould which is xe2x89xa742xc2x0.
Preferably, said base profile of the sector, comprising said central portion and said flanking connecting portions, has a camber f of xe2x89xa67.5 mm.
A third aspect of the invention is a mould for producing a tire from a predetermined viscoelastic material, said mould having
i. a crown formed by radially movable sectors for moulding a tread strip and shoulders of said tire, and
ii. cheeks for moulding sidewalls and beads of said tire,
iii. said mould having a profile, in the plane of the meridian section, which has a predetermined maximum chord C, a predetermined maximum height H and a fitting line I, and is formed centrally by a base profile of one sector and laterally by profiles of said cheeks,
iv. said base profile of the sector having a central portion flanked by two connecting portions, which in turn are flanked by two lateral portions, characterized in that
v. said central portion of said base profile of the sector is substantially flat and has a radius of meridian curvature Rxe2x89xa7500 mm, and
vi. each of said connecting portions has, at the point of junction with one of said lateral portions, an angle xcex10 with respect to the longitudinal axis of the mould which is xe2x89xa742xc2x0.
Preferably, said base profile of the sector, comprising said central portion and said flanking connecting portions, has a camber which has the value indicated above.
The mould according to the invention can be used to produce a tire with the desired inflated configuration, as a result of the fact that the base profile of each sector, in other words the envelope line at the base of its projections and grooves, has at its sides an angle of inclination with respect to the axis of rotation of the tire which is very small, or in any case is smaller than that of a conventional mould. The base profile of the sectors is therefore flatter and has a more open outlet than a conventional mould. In turn, the cheeks of the mould have radii of curvature Rfi and Rfs which have a characteristic ratio Rfs/Rfi in the range from 0.45 to 0.56, and preferably ≅0.5, and their centre line is located at a height Hlc equal to approximately ⅔ of the height of the cheek Hg. Additionally, the cheeks have a maximum chord C which is wider, and a width at the rim E which is greater than in a conventional mould, to prevent the beads of the tire from being moulded in a configuration which is too xe2x80x9cinsetxe2x80x9d, in other words too inclined with respect to the axis of rotation of the tire, and therefore from having a low rigidity in the inflated state.